There are two main causes of this problem, the gearbox not selecting a gear (ie. being stuck in neutral) or the bushing in the hub of the propeller having failed allowing the propeller shaft to spin but the propeller to slip and not turn as quickly as the shaft. In neutral, the boat will go nowhere, if the bushing is failing on the propeller the boat will generally move, but much more slowly than expected for the given RPM.

Spun propeller bushing

This topic covers the situation where the propeller is not spinning at the same speed as propeller shaft, therefore reducing forward thrust.

The gearbox doesn't go into gear when shifting the lever

If, when you shift the gear lever the gearbox doesn't go into gear, this is the section to look at.